The present invention generally relates to a laser printer. In particular, the invention concerns a control of a picture, pattern or image signal in the laser printer.
In the laser printers in general, a laser beam produced by a laser device is modulated in accordance with information to be printed, whereby a rotating light-sensitive drum is scanned with the modulated light beam through a scanner to thereby produce a corresponding latent image on the drum surface. Subsequently, the latent image is developed on the drum surface by means of developing toner. The toner image thus developed is then transferred to a recording sheet. The scanner is usually constituted by a polyhedron mirror and rotated at a high speed. An examplary structure of such laser printer is disclosed in U.S. Pat. No. 3,750,189. The development process is generally classified into two types. According to the first type of development, those portions which are irradiaed with the light beam are printed in black with toner. On the other hand, according to the second type development, those portions which are not irradiated with the light beam are printed. The latter type development is referred to as the positive development. The present invention can be very advantageously applied to the laser printer of the positive development type.
FIG. 1 illustrates a raster scan pattern on the drum surface in the positive development process. Reference numeral 55 designates irradiation with a correspondingly modulated laser beam on a single line scan base. Thus, the raster is constituted by traces of the line scans. Due to the modulation with printing information, i.e. information or data to be printed, there is produced an interrupted portion 56 which is not irradiated with light. Although the scanner itself is finished with a high precision, there may be produced an offset raster line such as indicated by A in FIG. 1 due to even a slight tilting of a mirror facet, because the raster line is of a very narrow width. As a result, a portion B which should inherently be irradiated with light remains non-irradiated, involving a stripe in black on the recording sheet after the development, to degrade the print quality. An attempt to enhance further the precision of the scanner with a view to overcoming such difficulty will incure a bloated expenditure, rendering the scanner impractical.
On the other hand, when the laser output is decreased due to deterioration of the laser device, diameter of the laser beam is correspondingly reduced, which results in that the width of the raster or scan line 55 becomes narrower to increase the raster line space, as illustrated in FIG. 2. Under the circumstances, thin lines will make appearance on the recording sheet after development.